The objective is to define immunological factors determining genetic resistance and susceptibility to Rous sarcomas in the natural host, the chicken. Using highly inbred resistant and susceptible chickens, studies will be performed to: a) determine the number of loci which control the dominant resistance trait and determine linkages with MHC, Ig and B-T cell alloantigens, b) analyze procedures which alter genetic resistance, c) define the role of MHC in response to tumor antigens, d) determine the effect on genetic resistance of autoimmunity to idiotypes of antibodies to virion and non-virion tumor antigens. Methods to be employed include surgical and hormonal bursectomy, indirect fluorescent antibody technique, use of iodinated protein A as an Ig fragment probe, in vitro assays for cell-mediated immunity based on infective center inhibition, employment of immunoadsorbents and elution for purification of antibody, characterization of antibodies by specificity, class and function, and in vivo measurements of tumor development.